The present invention relates to high current electrical interruption devices and, more particularly, to arc chutes employed in such devices.
Present-day equipment employed in power distribution systems is often required to interrupt large current flows. These devices, such as circuit breakers, must act repeatedly as opposed to fuse-type devices which act once to interrupt the current and then are discarded and replaced. Because of inherent inductive effects, the interruption of high current levels, often in excess of 1,000 amperes, generally results in an electric arc being struck between the separating portions of the main contact pair. The presence of this arc, through hot ionized plasma between the contacts, produces two undesirable results. First, the presence of the arc means that current flow is not instantaneously interrupted and this means that for a certain period of time following actuation of the drive mechanism which separates the contacts, there is a certain level of current flowing in the circuit which the circuit breaker is operating to protect. Second, the presence of the arc between the main contacts often results in erosion, pitting and general deterioration of the main contact surfaces. As a result, relatively high resistance regions on the contact surfaces can develop. Furthermore, if arcing is extensive, it is also possible under certain circumstances to heat the main contacts to a temperature sufficiently high so as to cause melting, and subsequent circuit breaker operation is impaired or inhibited. Accordingly, it is seen that mechanisms for minimizing the presence of electrical arcs between the main contacts are highly desirable.
One of the structures that is employed to assist in the removal of the arc struck between the main contacts is a device known as an arc chute. These devices have been employed in the past. These previously-employed arc chutes comprise stationary metal plates aligned substantially parallel to one another. In general, the arc between the main contacts is driven from the main contacts to the arc chute plates by magnetic forces or by gaseous forces. The arc then generally proceeds to travel in multiple arc paths (arclets) between the arc chute plates. However, decaying plasma between the main contacts may permit the arc to restrike across the main contacts and to move back out of the arc chute. Additionally, the number of arc chute plates between which the arc resides is also typically variable and difficult to predict for a particular arc chute design and even for multiple strikes with a single design.
The unpredictability of the number of arcs between the plates of the arc chute gives rise to a number of disadvantages for the conventionally-employed arc chute design. In particular, the voltage that the circuit interruption device is able to withstand subsequent to current interruption is proportional to the number of gaps in which arcing was occurring prior to total current cessation. Accordingly, it is seen that it is highly desirable that a sufficient number of gaps be provided together with the ability to ensure that arcing between each gap does, in fact, occur. Additionally, the ability of the circuit breaker to limit current is also proportional to the number of arcing gaps. Thus, the general, one would like to employ an arc chute having a large number of gaps. However, because of the limitation imposed by physical size, it is difficult to blow or otherwise transfer an arc between the main contacts and a high plate stack. The larger the number of gaps, the larger the number of plates which must be provided to form these gaps. Again, the larger the number of plates that are required, the higher the stack that is required. However, as was pointed out, transfer of the arc between the main contacts to such a long stack is impractical, if not impossible.